Powderless etching bath and method for etching curved segments



United States Patent Delaware No Drawing. Filed Aug. 27, 1964, Ser. No.392,589

Claims. (Cl. 252-794) This invention relates to etching metal. Moreparticularly it relates to a new and novel bath composition and methodconcerning the powderless etching of curved magnesium base metal workpieces such as curved etchable photoengraving plates and the like.

The art of etching flat plates or work pieces by powderless etchingprocesses has been developed so as to be acceptable to most users. Ingeneral, most of the conventional powderless etching bath compositionscomprise a mineral acid; a filming agent; and a water-immiscible organicfluid. U.S. Patent 3,023,138 is illustrative of such a bath, employingparticular filming agents, specifically usable for fiat plate etchingand comprises: nitric acid with the optional inclusion therewith ofsmall amounts of sulfuric acid, hydrochloric acid, acetic acid and thelike; an organic water-immiscible solvent stable in the presene ofdilute acids, and a substituted halogenated diaryloxide sulfonatewherein the substituents on halogenated diaryloxide sulfonates arehydrophobic groups containing up to 30 carbon atoms; the balance beingessentially water including, for example, various by-products generatedfrom the bath components.

In addition, so-called film controlling agents have been added to theabove bath formulations in an attempt to improve their performance. Onesuch group of film controlling agents comprises, for example, a singleorganic dicarboxylic acid or a blend of such compounds whichindividually consist of only hydrogen, carbon, and oxygen and have from4 to 10 carbon atoms with up to 3 carbon to carbon double bonds, thelatter referring to aromatic unsaturation as well as that which mayoccur in aliphatic chains. Such additaments may be aromatic,cycloaliphatic or aliphatic in character or a combination thereof suchas alkylaromatics. By their use in the above described etching baths, itis possible to increase the depths of etch obtainable in small areas ofpenetration in combination of photoengraving plates, such as halftoneareas where proper or adequate depths of etch are usually difiicult toobtain by bath modulation without, at the same time, adversely weakeningthe protection achieved in open line areas of the same plate. Inaddition, the film-controlling agents have a beneficial stabilizingeffect on the protective film in open line areas of the plate therebyminimizing lateral etching and chipped or rough image sidewalls.

Notwithstanding the aforesaid improved bath compositions, however, whenthey are applied to etching curved plates, particularly those exceedingabout 5 inches in diameter, severe non-uniform image shoulders commonlyresult causing in many cases loss of the image due, for example, tochipping along the edges. In addition, excessively wide or elongatedimage shoulders or tails (this affect being termed herein as tailing)are usually encountered in the direction of curvature. Conversely, thetransverse shoulders are frequently exteremely tight, i.e., essentiallyvertical, and in severe cases undercutting of the image itself mayoccur. Furthermore, engraved curved plates which are dirty, i.e., havingpimple-like projections in the non-image areas, are common. Thisrequires an additional step in the overall operation, which may beaggravated if the number of such pimples ice is excessive, of removingthem to obtain a clear plate. These undesirable results, either incombination or alone, cause poor quality in the reproduced image, andwhen severe, frequently results ultimately in the entire plate beingscrapped.

Though various etching machines have been developed in an attempt toimprove etching of curved segments, an example of such a machine beingdescribed in U.S. Patent 3,122,150, these machines do not significantlynor routinely avoid the heretofore mentioned problems, nor has anysatisfactory bath composition yet been devised for use with thesemachines for specifically etching curved plates which consistentlyavoids these diificulties.

Accordingly, a bath composition specifically designed for etching curvedwork pieces which essentially avoids the foregoing difficulties isneeded in the art.

An object of the present invention, therefore, is to provide a new andnovel etching bath composition and method specifically for powderlessetching of magnesium and magnesium base alloy curved segments, whichsignificantly minimizes the disadvantages herein before mentioned withrespect to etching such work pieces.

Another object of the invention is to provide a novel bath solutionprimarily for etching curved magnesium and magnesium base alloyphotoengraving plates of large diameter which substantially avoidschipping, tailing of the image shoulder and dirty, pimply plates, andwhich improves the uniformity, quality and cleanliness of the plate andrelief images in general.

Still another object is to provide a novel method of powderless etchingcurved work pieces.

It has now been found that by substituting a particular three componentorganic solvent system for the waterirnmiscible organic fluids used inthe here-inbefore described flat plate baths, a new and novel bathformulation is obtained in accordance with the present invention whichwhen used to etch curved segments substantially avoids the aforesaiddifficulties of tailing, bleed off, chipping and undercutting of theimage. Moreover, a clean photo engraved plate is obtained wherein thenumber of pimplelike projections in the non image areas have beenmarkedly reduced, if not substantially eliminated.

This three component organic solvent system comprises three groups oforganic liquids, each group being identified by a certain range of KauriButanol values (termed herein as KB values). The first group is calledthe low KB solvent having a KB range of from about 25 to about 75, thesecond group being called the intermediate KB solvent having a KB rangeof fro-m about to about 100, and the third group called the high KBsolvent having a KB value in excess of 100. The term KB value as usedherein refers to a known and used standard test for determining thesolvent power value of a given organic liquid as the amount of a solventwhich can be added to a standard kauri gum solution in butanol toproduce a definite turbidity, as compared with the amount of purebenzene used as a standard in a similar titration and arbitrarily takenas percent standard.

Required properties of the low KB solvent are that it have a KB valuewithin the range of from about 25 to about 75, be substantially stablein the presence of dilute nitric acid at bath temperatures, have somesolvency for the filming agent employed and that it be substantiallywater-immiscible. Substantially stable is defined as the non-occurrenceof deterioration, within a reasonable time period, in contact withdilute nitric acid which deterioration significantly alters the functionof the organic waterimmiscible component within the bath in anunfavorable manner. Also, it is essential that this additive be a liquidat bath temperatures. Suitable organic materials which can be usedindividually, or in combination with other such solvents within thetotal amount of such solvent as specified below, are aromatics,aliphatics, napht'henes, ethers, and esters having a KB value Within therange of from 25 to 75. Such low KB solvents include, for example,dodecylbenzenes, (an alkylbenzene, wherein the average of the alkylchains contains 12 carbon atoms), mineral seal oils, otheralkylbenzenes, napthenes with alkyl carbon atoms with the inclusiverange of from about 8 to 10, and the like. Generally, it may be saidthat water-immiscible esters, ketones, terpenes, ethers, aliphatic,naphthenic, and aromatic hydrocarbons which have a KB in the range of 25to 75 are operable.

The total amount of the low KB solvent employed may range from about 0.5to about 2 volume percent of the bath and preferably from about 0.75 toabout 1.25 volume percent. A particularly desirable amount to use hasbeen found to be about 1.0 percent.

The intermediate KB solvent constituent is present in the present novelbath formulation for the purpose and function, among other things, ofaiding in the control of the transverse shoulder width, in general, andmore specifically to avoid tightening and under-cutting of the shoulderin the transverse direction of the curve of the plate. This organicliquid is a solvent having with respect to the other organic solvents inthe bath as intermediate KB value, for example, from 75 to 100. Examplesof such organic liquids suitable for use in the present invention are:Solvesso 150 (a trade name for a mixture of approximately 90 percentalkylbenzenes, 2 percent napthalene and 8 percent napthenes);diethylbenzenes, such as, for example, Penola H.A.N. (a trade name foran aromatic solvent containing 84 percent of such aromatics having aflash point of 140 F. and initial boiling point of about 340 F.) andother aromatic and alkyl-aromatic hydrocarbons having a KB value withinthe above range of 75 to 100. The amount of such organic liquid to beused individually or in mixture thereof will be within the range of fromabout 0.8 to about 2.8 percent by volume of the composite bath.Preferably, the amount Will be within the range of from about 1.0 toabout 1.8 volume percent, said Solvesso 150 being preferred as theorganic liquid to be used.

The high KB, low viscosity, solvents are characterized by theirimmiscibility with water and a relatively high KB value, preferablywithin the range, for example, of from about 100 to about 130, but alsoincluding those having a KB value in excess of 130, such as, forexample, hexylacetate which has a KB value of about 203. In additionthey are substantially unreactive with the mineral acid component(dilute as used in the bath) and contain no terminal functional groupsbut rather a preponderance of carbon and hydrogen atoms. These solventsmay be used separately or in mixture within the total amount of saidhigh KB solvent to be used, as specified below and have the primaryfunction in the present bath of aiding in clean liness of etch, i.e.,avoidance of pimples on the etched plate without causing tailing on theimage shoulders of half-tone dots, or bleed off as it is called hereinsince it concerns more particularly the shoulders of half-tone highlightdot type images. The total amount of this high KB solvent to be used inthe bath ranges from about 0.25 to about 1.5 volume percent andpreferably from about 0.4 to about 0.6 volume percent. A particularlysuitable amount has been found to be about 0.5 volume percent. Examplesof such high KB solvents include hexyl acetate, dibutyl phthalate,trichloroethylene and the like. A very desirable high KB solventpreferred for use herein is a group of high boiling aromatic naphthasolvents known under the trade name of Panasol AN petroleum solvents,having high flash points (190230 F.), particularly those designated asAN-l made and identified as such by the Amoco Chemicals Corporationwhich are a group of relatively non-toxic, low volatile, aromaticnapthas of wide boiling range, having a KB value of about 108 andcontaining typically 99 percent of said high boiling aromatics.

In practicing the invention the novel three component, water-immiscible,organic solvent is admixed with the dilute nitric acid, a filming agentand a film controlling agent, all as previously described, the balancebeing essentially water, to form an etching bath. This novel bath isthen impinged upon the surface to be etched, such impingement beingaccomplished as by spraying, splashing, bubbling agitated immersion, orthe like. Etching is continued until the desired depth, which may be,for example, from about 25 to about 40 thousandths of an inch in large,open, non-image areas, is achieved.

The average bath temperature may be within a range from 40 to 120 F. buta preferred range is from to 90 F., and more preferably from to 85 F.

In formulating the etching bath, it is a general rule that as theconcentration of the nitric acid is increased within the aforementionedlimits, it is usually desirable to also increase the proportion offilming agent employed. Also, as will be manifest to those skilled inthe art, it may become desirable during the use of the bath to replenishthe acid that has been used up and possibly to add other agents such asbath extenders for special purposes.

The metals which may be etched by the process of this invention includeprimary magnesium, and magnesium base alloys, which are substantiallyhomogeneous and suitable for photoengraving. A magnesium base alloy isdefined as having percent of the base component. The bath whileselectively forming an acid-resistant film on the plate to protect theresist .and relief side walls or shoulders from lateral etching,substantially avoids formation of excessively elongated shoulders asheretofore discussed, thus producing a quality etched curvedphotoengraving plate to a degree heretofore unobtainable.

A preferred bath composition of the present invention for useparticularly in an etching machine adapted to etching curved platescomprises from about 88 to about 107 grams per liter of nitric acid;from about 1.0 to about 1.8 volume percent of Solvesso .150 (definedhereinbefore) as the intermediate KB solvent; from about 3.2 to about3.6 grams per liter of bath volume percent of a filming agent comprisinga mixture primarily of the sodium salts of mono and disulfonatedmonochlorododecyl, mostly C12, diphenyloxide with a preponderance ofmonosulfonated oxide (termed herein as diphenyloxide mixture); fromabout 1.2 grams per liter of bath to about 1.8 grams per liter of bathof adipic acid as a film controlling agent; from about 0.75 to about1.25 percent by volume of the low KB solvent dodecylbenzene; and fromabout 0.4 to about 0.6 percent by volume of Panasol AN-l as the high KBsolvent.

Though from 88 to 107 grams per liter of nitric acid is preferred in thebath composition as stated above, it is to be understood that from about78 to about 116 grams per liter of bath of said acid may also operablybe employed. Similarly, it is to be understood that from about 2.5 toabout 4.0 grams per liter of bath of the filrn'ng agent may also beoperably employed. A particularly desirable group of filming agents touse comprise a mixture of primarily the sodium salts of mono anddisulfonated monochlorododecyl, mostly 1-2 carbons, diphenyloxide with apreponderance of monosulfonated oxide, both the pure compounds andmixtures thereby being termed herein as diphenyloxide filming agents.Operable amounts of the film controlling agents which may be 7 usedinclude from about 0. 3 to about 2.5 grams per liter of bath of saidagent. The film controlling agents may be, for example, single organicdicarboxylic acid or a blend of such compounds which individuallyconsist of only hydrogen, carbon and oxygen and have 4 to 10 carbonatoms with up to 3 carbon to carbon double bonds, the latter referringto aromatic unsaturation as well as that which may occur in aliphaticchains. Such additaments may be aromatic, cycloaliphatic or aliphatic incharacter or a combination thereof such as alkylaromatic. Adipic acid isa preferred film controlling agent.

The following examples further illustrate the present invention but arenot intended to limit thereof:

Example I A powderles-s etching bath of 77 liters was prepared for usein etching curved photoengraving plates in a M-32 master etching machineby admixing; 9.93 grams per liter of HNO 0.335 grams per liter of bathof the aforesaid diphenyloxide mixture (both defined hereinbefore); 1.34percent by volume of Solvesso 150; 1 percent by volume ofdodecylbenzene; and 0.5 volume percent of Panasol AN-l, and 1.5 gramsper liter .of adipic acid. The balance of the bath was made up withwater. The bath temperature was then adjusted .to 80 F. A 11 inchdiameter by 18 inch long curved segment 0.064 inch thick of an etchablemagnesium base alloy phot-oengraving plate exhibiting a polyvinylalcohol type etch resist coating as the image pattern and bare metal inthe non-image areas (the areas to be etched) was then placed in positionfor etching in the machine by clamping it onto a mandrel. The machinewas then operated whereupon, the paddles were turned and the preparedbath compositions splashed on the segment as it turned. The mandrelspeed was about 40 rpm, and the direction of rotation was reversed each20 seconds. Etching was continued for about minutes to a depth of about.030 inch. The depth of the etch in the high-light dot areas of the 65line screen halt-tone areas was about 0.005 inch.

An examination of the etched segment showed an absence of excessivetailing and bleed off. All the image shoulders were observed to besignificantly more uniform, and chip free, and at an overall angle ofapproximately 45 to the horizontal. This plate was suitable for directprinting purposes.

Example [I To illustrate the present invention on a comparative basis,an 8 inch diameter curved segment was etched in a powderless etchingbath in the same machine and under the same conditions as used inExample I, except that a lower bath temperature of about 70-71 F. wasemployed, and the etching time was increased to give the same open areadepth of about 0.030 of an inch. This standard bath contained 94 gramsper liter of HNO 3.3 grams per liter of a diphenyloxide filming agent,1.5 grams per liter of adipic acid, and 45 volume percent of Solvesso150. Subsequently, an examination of the plate segment showed severetailing and bleed oif (bleed off being a form of tailing as related tohalf-tone highlight dots) of the image shoulders in the direction of thecurvature. The transverse shoulders, that is, those at 90 angles to theshoulders in the direction of curvature, were extremely tight, that is,essentially vertical, and in some cases undercutting of the image itselfwas evident. In order to determine more objectively the severity of thetailing of the image shoulders, the distance from the top edge of theimage shoulder to where the shoulder meets the relief surface wasmeasured and the figure so obtained divided by the depth of the totaletch on the plate, to give a quotient which was the tangent of theshoulder angle. By reference to trigonometry tables the shoulder angleWas then determined. The procedure was then repeated but with respect tothe transverse shoulder. The angle obtained from this latter transverseshoulder measurement was'then subtracted from the former to obtain anangle of difference which indicates generally the quality of the overallshoulder profile. In so doing an angle of difference of about 59 wasobtained indicating that the image shoulder was excessively elongated inthe direction of curvature. In addition several pimples in the nonimageareas were present.

The foregoing etching and examination procedure was repeated using thesame curved plate size, equipment, and etching conditions except that inthis case the new and novel bath composition of the present inventionfor etching curved plates was employed at a bath temperature of about F.The angle of difference obtained was 34, indicating that significantlymore uniform image shoulders were produced, thus a markedly higherquality plate was made. Moreover, the plate was generally cleaner andexhibited no pimples in the non-image areas, thus was suitable fordirect printing purposes.

Combinations of bath additives with the present invention other thanthose hereinbefore specified may also be used to constitute other novelbath composition formulations within the present invention with the samegood results in etching curved plates, with respect to significantlyreducing tailing and bleed-01f aflects. Examples of such othercombination include:

(a) The same composition as in Example I except that 2 percent Solvesso150 and 2 percent dodecylbenzene was used including 0.5 percent ofPanasol AN-2 instead of AN-l.

(b) The same composition as in Example I except that 1 percent Solvesso150 was used and also a mixture of 1 percent diisodecylphthalate and 0.5percent Panasol AN-l as the high KB solvent.

(0) The same composition as in Example I except that /2 percent ofSolvesso 150 was used and also a mixture of A percentdiisodecylphthalate and 1 percent Panasol AN-l as the high KB solvent.

It is manifest that various modifications can be made in the process ofthe present invention without departing from the spirit or scope thereofand it is understood that I limit myself only as defined by the appendedclaims.

What is claimed is:

1. In an etching bath composition comprising substantially nitric acid,a sulfonated dodecyl diphenyl oxide filming agent, a film controllingagent and an organic water-immiscible fluid substantially stable in thepresence of dilute nitric acid, the balance being essentially water; theimprovement consisting essentially of employing a three-componentorganic solvent system as the waterimmisci-ble fluid, said systemcomprising as a first component from about 0.5 to about 2.0 volumepercent of the bath of at least one organic liquid having a kauributanolvalue within the range of from about 25 to about 75, as a secondcomponent from about 0.8 to about 2.8 volume percent of the bath of anorganic liquid having a kauri-butanol value within the range of fromabout 75 to about 100, and as a third component from about 0.25 to about1.5 volume percent of the bath of an organic liquid having akauri-butanol value in excess of 100.

2. The improved bath of claim 1 wherein the third component organicliquid has a kauri-butanol value within the range of from about 100 toabout 130.

3. The improved bath of claim 1 wherein the first component organicliquid is an alkyl benzene.

4. The improved bath of claim 3 wherein the alkyl benzene isdodecylbenzene.

5. The improved bath of claim 1 wherein the second component organicliquid is a mixture of approximately percent alkylbenzenes with about 2percent of naphthalene and 8 percent naphthenes.

6. The improved bath of claim 1 wherein the third component organicliquid is a high boiling, substantially non toxic, aromatic naphthasolvent having a high flash point.

7. The improved bath of claim 6 wherein, the aromatic, high boiling,naphtha has a kauri-butanol value of about 108.

8. The improved bath of claim 1 wherein the mineral acid is nitric acid.

9. An aqueous bath composition for etching etchable curvedphotoengraving plates consisting essentially of from about 88 to about107 grams per liter of nitric acid; from about 3.0 to about 3.6 gramsper liter of bath of a sodium salts of mono and disulfonatedmonochlorododecyl diphenyl oxide filming agent; from about 0.75 to about1.25 volume percent of the bath of dodecyl benzene; from about 1.0 toabout 1.8 volume percent of the bath of a mixture of approximately 90percent alkyl benzenes, 2 percent naphthalene and 8 percent naphthenes;from about 0.4 to about 0.6 volume percent of the bath of a high boilingaromatic naphtha solvent having a kauri-butanol value of about 108; fromabout 1.2 to about 1.8 grams per liter of bath of adipic acid as a filmcontrolling agent; the balance being essentially water.

10. A method of etching etchable curved photoengraving plates ofmagnesium base metal having an acid resistant image on the surfacesthereof to be etched comprising; (a) establishing an aqueous powderlessetching bath composition comprising from about 88 to about 107 grams perliter of nitric acid, from about 2.5 to about 4.0 grams per liter ofbath of a sodium salts of mono and disulfonated monochlorododecyldiphenyl oxide filming agent; from about 0.5 to about 2 volume percentof the bath of at least one organic liquid having a kauri-butanol valuewithin the range of from about 25 to about 75; from about 0.8 to about2.8 volume percent of the bath of at least one organic solvent having akauri-butanol value within the range of from about 75 to about 100; fromabout 0.25 to about 1.0 volume percent of the bath of at least oneorganic solvent having a kauri-butanol value in excess of 100; fromabout 1.2 to about 1.8 grams per liter of at least one film controllingagent, the balance being essentially water; and (b) impinging said bathcomposition upon the surfaces to be etched of said photoengraving, saidbath composition being at a temperature of from about 65 to 85 F. untilthe desired depth of relief is obtained.

References Cited by the Examiner UNITED STATES PATENTS 2,318,559 5/1943Percival 252l42 X 3,023,138 2/1962 Easley et a1. 25279.4 X 3,152,08310/1964 Croisant et a1. 25279.4

LEON D. ROSDOL, Primary Examiner.

ALBERT T. MEYERS, Examiner.

SUSAN E. DARDEN, M. WEINBLATT,

Assistant Examiners.

1. IN AN ETCHING BATH COMPOSITION COMPRISING SUBSTANTIALLY NITRIC ACID,A SULFONATED DODECYL DIPHENYL OXIDE FILMING AGENT, A FILM CONTROLLINGAGENT AND AN ORGANIC WATER-IMMISCIBLE FLUID SUBSTANTIALLY STABLE IN THEPRESENCE OF DILUTE NITRIC ACID, THE BALANCE BEING ESSENTIALLY WATER; THEIMPROVMENT CONSISTING ESSENTIALLY OF EMPLOYING A THREE-COMPONENT ORGANICSOLVENT SYSTEM AS THE WATERIMMISCIBLE FLUID, SAID SYSTEM COMPRISING AS AFIRST COMPONENT FROM ABOUT 0.5 TO ABOUT 2.0 VOLUME PERCENT OF THE BATHOF AT LEAST ONE ORGANIC LIQUID HAVING A KAURIBUTANOL VALUE WITHIN THERANGE OF FROM ABOUT 25 TO ABOUT 75, AS A SECOND JCOMPONENT FROM ABOUT0.8 TO ABOUT 2.8 VOLUME PERCENT OF THE BATH OF AN ORGANIC LIQUID HAVINGA KAURI-BUTANOL VALUE WITHIN THE RANGE OF FROM ABOUT 75 TO ABOUT 100,AND AS A THIRD COMPONENT FROM ABOUT 0.25 TO ABOUT 1.5 VOLUME PERCNET OFTHE BATH OF AN ORGANIC LIQUID HAVING A KAURI-BUTANOL VALUE IN EXCESS OF100.